'Build' error when nesting form attributes - ruby-on-rails

Question

I'm encountering an error when trying to create a new nested form.
I have 3 models:
class Child < ActiveRecord::Base
has_many :hires
has_many :books, through: :hires
end
class Hire < ActiveRecord::Base
belongs_to :books
belongs_to :children
accepts_nested_attributes_for :books
accepts_nested_attributes_for :children
end
class Book < ActiveRecord::Base
has_many :hires
has_many :children, through: :hires
belongs_to :genres
end
I'm trying to set up a view which allows children to 'hire' 2 books.
The view looks like this:
<%= form_for(#hire) do |f| %>
<%= hires_form.label :child %><br>
<%= hires_form.select(:child, Child.all.collect {|a| [a.nickname, a.id]}) -%>
<%= f.fields_for :books do |books_form| %>
<%= books_form.label :book %><br>
<%= books_form.select(:book, Book.all.collect {|a| [a.Title, a.id]}) -%>
<%= books_form.label :book %><br>
<%= books_form.select(:book, Book.all.collect {|a| [a.Title, a.id]}) -%>
<% end %>
<%= f.submit %>
<% end %>
The controller looks like this:
class HiresController < ApplicationController
...
def new
#hire = Hire.new
2.times { #hire.books.build }
end
def create
#hire = Hire.new(hire_params)
respond_to do |format|
if #hire.save
format.html { redirect_to #hire, notice: 'Hire was successfully created.' }
format.json { render :show, status: :created, location: #hire }
else
format.html { render :new }
format.json { render json: #hire.errors, status: :unprocessable_entity }
end
end
end
...
private
# Use callbacks to share common setup or constraints between actions.
def set_hire
#hire = Hire.find(params[:id])
end
# Never trust parameters from the scary internet, only allow the white list through.
def hire_params
params.require(:hire).permit(:book, :child, books_attributes: [:id, :book, :child, :_destroy])
end
end
I'm getting the error:
undefined method `build' for nil:NilClass
I feel this is something obvious that i'm missing but any help would be great!

Answer 1

belongs_to :books
belongs_to :children
You need to singularize these.
belongs_to :book
belongs_to :child
Also see section 2.4 of the rails guides as a reference "The has_many :through Association":
http://guides.rubyonrails.org/association_basics.html#the-has-many-through-association
Keep in mind that you are using a belongs_to on the Hire object.
Notice the methods that are added when using the belongs_to association:
association(force_reload = false)
association=(associate)
build_association(attributes = {})
create_association(attributes = {})
create_association!(attributes = {})
So you should be able to use the method build_book on the Hire object (eg. #hire.build_book of #hire.build_child).
The methods added by the has_many association are:
collection(force_reload = false)
collection<<(object, ...)
collection.delete(object, ...)
collection.destroy(object, ...)
collection=(objects)
collection_singular_ids
collection_singular_ids=(ids)
collection.clear
collection.empty?
collection.size
collection.find(...)
collection.where(...)
collection.exists?(...)
collection.build(attributes = {}, ...)
collection.create(attributes = {})
collection.create!(attributes = {})
Which can be used on your Book and Child objects. For example: #book.childeren.build or #child.books.build.
For reference see the rails guides section 4.2 and 4.3 'Methods Added by ..':
http://guides.rubyonrails.org/association_basics.html#the-has-many-through-association